Shaped anode x-ray tube

ABSTRACT

An x-ray tube ( 16 ) suitable for use in a computed tomography (CT) scanner ( 10 ) includes an envelope ( 42 ) which defines an evacuated chamber. An anode ( 40 ) and a cathode assembly ( 70 ) are disposed within the chamber. The anode defines a target area ( 56 ) which is struck by electrons ( 52 ) emitted by a filament ( 54 ) of the cathode assembly and emits x-rays ( 46 ). The target area lies partially on a first annular portion ( 80 ) which is disposed at first angle (a) relative to a plane perpendicular to an axis of rotation (R) of the anode, and partially on a second portion ( 82,120 ) which is radially spaced from the first portion and disposed at a second angle (B), relative to the plane. The second angle is greater than the first angle. The portions of different slope allow the x-ray tube to take advantage of a shallow angle, while minimizing the heel effect.

The present application relates to the x-ray tube arts. The inventionfinds particular application in x-ray tube assemblies for large borecomputed tomography scanners. It is to be appreciated, that the presentinvention finds further application in other higher power x-ray deviceswhere it is desirable to increase the anode current without incurring aheat loading which is damaging to the anode.

Computed tomography (CT) scanners radiographically examine a subjectdisposed on a patient support and generate diagnostic images of thesubject. An x-ray tube assembly is mounted on a rotating gantry andprojects a beam of radiation through a section of the subject which isdetected by a detection system, such as an array of two-dimensionaldetectors which are mounted on the rotating gantry or a ring ofdetectors on the stationary gantry. To increase the width of the sliceor cone beam which is irradiated, the width of the detector array,parallel to the axis of rotation of the anode, has been progressivelyincreased. This increased width, in combination with faster scan times,places higher demands on the x-ray tube, in terms of generating a higherx-ray flux.

X-rays from conventional rotating anode x-ray tubes are typicallyemitted from a target on the sloped, peripheral edge of the anodetypically at a point nearest the patient, where the electrons strike andare converted to x-rays. The x-ray beam is typically collimated into afan or wedge of x-rays at an angle which is about 90° to the beam ofelectrons striking the anode. The peripheral edge is generally providedwith a slope to increase the target area at which a focused electionbeam strikes the anode, thereby decreasing the current loading per unitarea of the target. The width of the x-ray beam source (the focal spotwidth) is a projection of the height (radially) of the target area. Morespecifically, the projection is a function of the electron beam heighttimes the tangent of the angle of the slope of the peripheral face ofthe anode.

As a result of the demand for higher loadings, in recent years, theslope has decreased from about 10° (relative to an axis perpendicular tothe beam of electrons) to about 7°, or less. As seen from the tablebelow, this enables an increase of over 40% in anode current for thesame heat loading at a given projected focal spot size as viewed in thex-ray beam direction. Slope length (mm) Anode slope for a 1 mm (degrees)projection Relative loading 6 9.51 168 7 8.14 144 8 7.12 125 9 6.31 11110 5.67 100 11 5.14 91 12 4.70 83

At shallow angles (e.g., 7°), however, there is a tendency for the x-raybeam to be truncated or reduced in x-ray flux at the heel. Specifically,not all the incident electrons generate x-rays at the surface of theanode face. Rather, some electrons penetrate deeply within the targetbefore generating x-rays. X-rays generated at the surface do not passthrough the anode, provided the beam angle is not wider than twice thetarget slope. However, x-rays generated within the target must passthrough it and are attenuated by the heavy metal of the target. Theflatter the slope of the peripheral face and the wider the beam angle,the further the interior-generated x-rays must travel through the anodemetal before emerging in the direction of the output beam. The heeleffect attenuation is greater for x-rays on the anode side of the beam.

The CT scanner manufacturer is thus faced with the choice of specifyingeither an anode of shallow slope (e.g., 7°), which is limited in termsof the beam angle it can provide because of the heel effect, or ofsteeper slope (e.g., 10°), which is limited in terms of the loading itcan sustain.

The present invention provides a new and improved method and apparatuswhich overcome the above-referenced problems and others.

In accordance with one aspect of the present invention, an x-ray tube isprovided. The x-ray tube includes an envelope which defines an evacuatedchamber and a source of electrons. An anode is mounted in the chamberfor rotation about an axis of rotation. The anode defines a slopedperipheral region on which a target area is defined, which target areais struck by electrons emitted by the electron source and emits x-rays.The sloped peripheral region includes a first annular portion sloped ata first angle relative to a plane perpendicular to the axis of rotationand a second annular portion, adjacent the first, sloped at a secondangle relative to the plane. The second angle is different from thefirst angle. The target area is defined partially on the first annularportion and partially on the second annular portion.

In accordance with another aspect of the present invention, a method ofgenerating a beam of x-rays is provided. A beam of electrons isaccelerated and focused to strike a target area on a sloping peripheralregion of an anode which rotates about an axis of rotation. The anodeperipheral region includes a first annular portion sloped at a firstangle relative to a plane perpendicular to the axis of rotation and asecond annular portion radially spaced from the first and sloped at asecond angle. The second angle is different from the first. The targetarea is defined partially on the first annular portion and partially onthe second.

One advantage is that it enables an anode to have a shallow slope whilemaintaining a sufficiently large beam angle.

Another advantage of at least one embodiment of the present invention isthat it facilitates generating higher flux, wider x-ray beams.

Another advantage resides in reduced anode heating.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of a computed tomography scannerincorporating the present invention;

FIG. 2 is a partial cross-sectional view of one embodiment of an x-raytube of the computed tomography scanner of FIG. 1;

FIG. 3 is a detailed cross-sectional view of the anode of an x-ray tubeof FIG. 2;

FIG. 4 is a diagrammatic cross-sectional view of an anode and filamentcombination of another embodiment;

FIG. 5 is another diagrammatic view of a cross-section of anode andfilament combination; and

FIG. 6 is yet another diagrammatic, partial cross-sectional view of ananode and cathode filament combination.

With reference to FIG. 1, a computed tomography (CT) scanner 10radiographically examines and generates diagnostic images of a subjectdisposed on a patient support 12. More specifically, a volume ofinterest of the subject on the support 12 is moved into an examinationregion 14, typically by translating the support 12 in a direction Z. Anx-ray tube assembly 16 mounted on a rotating gantry projects one or morebeams of radiation through the examination region 14. A collimator 18collimates the beams of radiation in two dimensions. In the preferredembodiment, a two-dimensional x-ray detector 20 is disposed on therotating gantry across the examination region 14 from the x-ray tube. Inanother embodiment, a ring or array of two-dimensional detectors ismounted on a stationary gantry around the rotating gantry.

The x-ray detector 20 operates in known ways to convert x-rays that havetraversed the examination region 14 into electrical signals indicativeof x-ray absorption between the x-ray tube 16 and the detector 20. Theelectrical signals, along with information on the angular position ofthe rotating gantry, are communicated to a data memory 30. The data fromthe data memory 30 is reconstructed by a reconstruction processor 32.Various known reconstruction techniques are contemplated including conebeam, multi-slice, and spiral scanning and reconstruction techniques,and the like. The volumetric image representation generated by thereconstruction processor 32 is stored in a volumetric image memory 34. Avideo processor 36 withdraws selective portions of the image memory tocreate slice images, projection images, surface renderings, and the likeand reformats them for display on a monitor 38, such as a CRT or LCDmonitor.

With reference now to FIG. 2, the x-ray tube assembly 16 includes adisk-shaped anode 40, which is mounted within an air-evacuated envelope42 and may be in a plane perpendicular to the axis of rotation of therotating gantry, although other geometries are also contemplated. Theevacuated envelope is surrounded with a lead or another high-Z metalwith good x-ray stopping power housing 44 which defines a coolingreservoir. A window 45 of beryllium or other low-Z metal or materialdefines an exit near the examination region 14 through which x-rays 46enter the examination region 14. Situated between the examination region14 and the window 45 is a beam-shaping filter (not shown) and thecollimator 18.

The anode 40 has a sloped, annular peripheral edge 50 which is struck bya beam 52 of electrons generated by a source of electrons, such as afilament 54 of a cathode assembly. The beam of electrons is focused tostrike a limited, defined area or target 56 on the sloped edge. Theanode is mounted on a central shaft 58 and rotates about an axis R,which is generally parallel with the beam of electrons 52 andperpendicular to a front face of the anode. The sloped target 56 isspaced from the axis R by a distance d₁ at its inner peripheral edge 60and by a distance d₂ at its outer peripheral edge 62. The majority ofthe electrons in the beam 52 strike the anode in the target 56, withonly a minimal proportion striking other parts of the anode surface. Thetarget 56 preferably receives at least 90% of the electrons which areemitted by the cathode and which hit the anode, more preferably, atleast about 99% of these electrons.

The filament 54 is mounted in a cathode cup 70, which acts as a focusingdevice to focus the electrons .emitted by the filament into the beam 52which is accelerated by a high voltage source 72 to the anode. Thecathode cup and filament, which together make up a cathode assembly,remain stationary, with respect to the envelope 42, although it is alsocontemplated that the cathode assembly may rotate while the anoderemains stationary. In any event, the cathode assembly remainsstationary with respect to the output beam 46.

With continued reference to FIG. 2, and reference also to FIG. 3, thetarget 56 is defined partially on a primary portion 80 of the peripheraledge 50 and partially on a secondary portion 82 of the peripheral edge.The secondary portion 82 is located radially inward of the primaryportion 80. The primary portion 80 extends at an angle a to a planewhich is perpendicular to the axis R of the anode. The secondary portionextends at an angle β to an axis which is perpendicular to the axis R ofthe anode. Angle β is larger than angle α. In one embodiment, the anglesα and β differ by at least 1°. In another embodiment, the angles differby at least 2°. For example, angle α is from about 6° to about 8°, whileangle β is from about 8° to about 12°. In one specific preferredembodiment, the angle α is about 7° and the angle β is at least about9°, preferably 10°. The lower limit of the angle α depends on thedetectors, the resolution, and the width of the beam desired. Incurrently available CT systems, these do not allow an angle α of muchless than 6°, although it is contemplated that advances in CT scannertechnology may permit smaller angles.

In the preferred embodiment, the majority of the electrons which strikethe target 56 strike in the primary portion 80. In one specificembodiment, at least about 60% of the electrons which strike the target,strike the primary portion 80, with the balance of 40%, or less strikingthe secondary portion 82. Preferably, at least 80% of the electronsstriking the target 56 strike one or other of the primary and secondaryportions, more preferably, at least 90%. In FIG. 3, the primary portion80 is shown as ending abruptly as the interface with the secondaryportion 82, although it preferably does not do so, as discussed below.

The combination of the primary portion 80 with the secondary portion 82allows for a high power, due to the shallow angle of the primaryportion, while reducing the heel effect with the secondary portion. Theprojection p₁ of the x-ray beam from the primary portion 80 is relatedto the height h₁ of the electron beam striking the primary portion bythe expression:P₁=h₁ tanα

and similarly for the secondary portion 82:p₂=h₂ tanβwhere P₂ and h₂ are the projection and height, respectively, of thesecondary portion. It will be appreciated that h₁ and h₂ may be lessthan or equal to the actual heights of the primary and secondaryportions, where the electron beam width w does not extend beyond theseportions. For this embodiment, where the first and second portions aredirectly adjacent, h₁+h₂=h_(T)=w.

With reference once more to FIG. 2, the filament 54 includes a firstportion 90 and a second portion 92. Due to the focusing effects of thecathode cup 70, the x-rays emitted by the first portion 90 predominantlystrike the primary portion 80 of the target; while the x-rays emitted bythe second portion 92 predominantly strike the secondary portion 82 ofthe target. The first portion 90 of the filament emits a higher currentthan the second portion 92. It will be appreciated that although thefirst filament portion 90 is shown as being axially aligned with theprimary target portion 80, and the secondary filament portion 92 alignedwith secondary target portion 82, in cathodes which includeinversion-type electronics, where the upper half of the filament isimaged on the lower half of the target, the relative positions ofportions 90 and 92 are reversed.

The larger current of the first portion 90 is readily achieved byproviding a larger coil diameter d₁ for the first portion 90 than thecoil diameter d₂ of the second portion 92. Other known methods ofproviding a larger current are also contemplated. The x-ray flux emitted(photons per unit area) is thus lower for the secondary target portion82 than for the primary target portion 80. To accommodate for anyvariations in the flux, the reconstruction processor 32 of the CTscanner (FIG. 1) is optionally programmed to take the variations in fluxinto account when reconstructing the image.

Preferably, the electron source is configured to deliver the same (or atleast substantially the same) specific load to the anode in all portionsof the target. Preferably, the specific load on the first annularportion is within ±10% of the specific load on the second annularportion. Specific load can be defined as the current (in mA) per unitarea (cm²) of the sloped surface.

The shaping of the filament exploits the shaping of the anode bydistribution the current load over its surface appropriately. When thefilament current is increased, the cathode emission will increaseproportionately at all points, and the image of the filament upon theanode will become uniformly brighter, with substantially unchanged ratioof the currents in its first and second portions 90 and 92.

In an alternative embodiment, the source of electrons 54 comprises twofilaments of helically wrapped wire or conductive film, a firstfilament, similar in dimensions to the first filament portion 90,emitting a first stream of electrons which are accelerated to strike theprimary target portion 80, the second filament, similar in dimensions tothe second filament portion 92, emitting a second stream of electronswhich are accelerated to strike the secondary target portion 82. Theoptimal relative heights of the target portions 80, 82 depends, in parton the CT scanner in which the x-ray tube is employed and in part on thedesired coverage. For example, a multislice CT scanner using 100 sliceswill generally benefit from a larger h₁/h₂ ratio than a 50 slice scannerof given width.

As shown in FIG. 3, portions 96, 98 of the anode surface adjacent thetarget area 56 are also sloped, relative to the beam direction. Theslope of these portions may be the same as that of the adjacent portion80 or 82 of the target, or the slope may be different.

The configuration of FIGS. 2 and 3 helps to alleviate the heel effect byproviding a region 82 of greater slope at the periphery of the primaryportion 80. Other embodiments which also provide for regions ofdifferent slope are shown in FIGS. 4-6, where similar elements are giventhe same numerals and different elements are given new numerals. Thex-ray tubes and anode configurations for these embodiments are the sameas for that of FIGS. 2 and 3, except as otherwise noted. It will beappreciated that in all the FIGURES, the angles α and β have been shownlarger than they are in practice for clarity and ease of illustration.

In the embodiment shown in FIG. 4, the primary target portion 80 isconnected with the secondary portion 82 by a smooth or curved transitionportion 110, which is tangential to the angle α adjacent the primaryportion 80 and is tangential to the angle β adjacent the secondaryportion 82. The curved portion 110 thus provides a gradual increase inthe angle of the target slope from α, adjacent the primary portion 80,to β, adjacent the secondary portion 82. The angles α and β can have thesame values as described for the embodiment of FIGS. 2 and 3 (e.g., 7°and 10°, respectively). In one embodiment, the curved portion 110 isabout 1-2 mm in height h₃ i.e., only a small proportion of the targetheight h_(T). For this embodiment, where the first and second portionsare spaced by the transition portion 110, h₁+h₂+h₃=h_(T)=w.

It will be appreciated that although the transition portion 110 is shownas being of similar length in primary and secondary to portions 80 and82, in practice, where the angles α and β are closer to the 7° and 10°discussed above, the curved portion preferably has a height h₃ which isshorter than height h₁ of the primary portion 80 and is optionallyshorter than the height h₂ of the secondary portion 82.

The coil 54 preferably transitions smoothly to match the transitionportion 110 of the target 56. As shown in FIG. 4, the filament coil 54has a width (diameter) d which is inversely proportional to tan θ(d=K/tan θ), where θ is the angle of the target at the point at whichthe electrons strike and K is a constant. Thus, for the first portion 90of the coil, which corresponds to the primary target portion 80, thewidth d₁=K/tanα and for the secondary portion 92, the width d₂=K/tanβ,as for the first embodiment. For a transition region 114 of the coilbetween the first and second portions 90, 92, the width graduallychanges, as a function of the tangent, tan θ. As for the firstembodiment, the reconstruction processor 32 is programmed to accommodatefor the change in flux which occurs as a result of the changing width ofthe filament coil 54.

An advantage of this embodiment is that the placement of the image ofthe filament on the anode need not be as precise as for the embodimentof FIGS. 2 and 3, to avoid variations in x-ray output. As x-ray tubebearings wear, the anode tends to suffer increasingly from anode wobble.Having the gradually curving transition portion 110 rather than a sharpchange between the primary and secondary portions 80 and 82 reduces theeffects of the anode wobble upon x-ray output, prolonging the usefullife of the x-ray tube.

With reference now to FIG. 5, another embodiment of an anode is shown.In this embodiment, the target 56 includes a first portion 80 having theslope α, as discussed above (e.g., 7°). A second portion 120 is curvedwith the curvature increasing, away from the first portion 80. In oneembodiment, the second portion transitions from the angle a at theintersection with the first portion and increases to the angle β at itsouter edge. β can be greater than 10°, for example, 12° or as high asabout 15°. The optimal value of β depends, to some extent, on the numberof slices used by the CT scanner. For larger numbers of slices a largerangle β is generally preferred. For example, for 50 slices, a β of 12°may be optimal, whereas for 100 slices, closer to 15° may be optimal forβ.

As with the other embodiments, the filament 54 is preferably shaped tomatch the change in slope of the target, with the width being generallydescribed by d=K/tan θ.

As with the embodiment of FIG. 5, this embodiment is less sensitive toanode wobble than that of FIGS. 3 and 4.

FIG. 6 illustrates an embodiment in which the flatter and more slopedregions are reversed in position. The target 56 slopes at an angle αnear the inside or top of the anode and progresses smoothly to an angleβ at the other end of the target area. In the illustrated embodiment,the cathode cup 70 is configured such that the filament 54 focuses amirror image on the target. The filament 54 again produces electrons ininverse proportion to the slope of the receiving face. Because theembodiment of FIG. 6 is becoming progressively steeper, the path lengththrough the anode traveled by x-rays which are generated below thesurface of the anode becomes progressively shorter reducing attenuationand heeling effect. Although shown as a continuous smooth curve, it isto be appreciated that the target area can be two linear segments, twolinear segments connected by a smooth transition region, a single linearsegment and a continuously curved transition region and secondaryregion, or the like. As another option, a dual filament can be providedsuch that the target area can be expanded from the illustrated region 56where the slope is between angles α and β, e.g. between 7 and 12°, andextended to a region where the slope is larger, e.g., 15°.

The invention has been described with reference to the preferredembodiment. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An x-ray tube Comprising: an envelope which defines an evacuatedchamber; a source of electrons; an anode mounted within the chamber forrotation about an axis of rotation, the anode defining a slopedperipheral region which a target area is defined, which target area isstruck by electrons emitted by the electron source and emits x-rays, thesloped peripheral region including a first annular portion, sloped atfirst angle relative to a plane perpendicular to the axis of rotation,and a second annular portion, adjacent the first portion, sloped at asecond angle, relative to the plane, the second angle being differentfrom the first angle, the target area being defined partially on thefirst annular portion and partially on the second annular portion. 2.The x-ray tube of claim 1, wherein the first annular portion is closerto a periphery of the anode than the second portion.
 3. The x-ray tubeof claim 1, wherein the first angle and the second angle differ by atleast 1°.
 4. The x-ray tube of claim 3, wherein the first and secondangles differ by at least 2°.
 5. The x-ray tube of claim 1, wherein thefirst angle is less than about 8°.
 6. The x-ray tube of claim 1, whereinthe first angle is from about 6° to about 8°.
 7. The x-ray tube of claim5, wherein the first angle is about 7°.
 8. The x-ray tube of claim 6,wherein the second angle is at least 8°.
 9. The x-ray tube of claim 8,wherein the second angle is about 10°.
 10. The x-ray tube of claim 1,further including: an annular transition portion intermediate the firstand second portions, the transition portion defining a smooth, curvedtransition between the first portion and the second portion.
 11. Thex-ray tube of claim 10, wherein the transition portion curves graduallyfrom the first portion to the second portion, the transition portionsloped at the first angle adjacent the first portion and sloped at thesecond angle adjacent the second portion.
 12. The x-ray tube of claim 1,wherein the second portion increases in slope with distance from thefirst portion.
 13. The x-ray tube of claim 1, wherein the first angle issmaller than the second angle, and the electron source is configured todeliver substantially the same specific load to the portion of thetarget area on the first portion than to the portion of the target areaon the second portion.
 14. The x-ray tube of claim 1, wherein the sourceof electrons includes a filament having a greater width in a region ofthe filament which emits electrons that strike the portion of the targetarea on the first annular portion and a smaller width in a region whichemits electrons which strike the portion of the target area on thesecond annular portion.
 15. The x-ray tube of claim 15, wherein thewidth of the filament varies such that the width is inverselyproportional to a tangent of an angle of a slope of a region of thetarget area that is struck by the electrons from the region of thefilament.
 16. A computed tomography (CT) scanner including the x-raytube of claim
 1. 17. The CT scanner of claim 16, wherein the CT scannerincludes at least one x-ray detector and a reconstruction processor, thereconstruction processor being programmed to account for a higher x-rayflux from the first annular portion than from the second annularportion.
 18. A method for generating a beam of x-rays, comprising:accelerating and focusing a beam of electrons; and striking a targetarea on a sloping peripheral region of an anode that rotates about anaxis of rotation, the peripheral region including a first annularportion sloped at first angle relative to a plane perpendicular to theaxis of rotation, and a second annular portion, radially spaced from thefirst annular portion and sloped at a second angle relative to theplane, the second angle being different from the first angle, the targetarea being defined partially on the first annular portion and partiallyon the second annular portion.
 19. The method of claim 18, furtherincluding: generating electrons such that a portion of the election beamwhich strikes the target area on the first annular portion has a greaterelectron current density than a portion of the election beam whichstrikes the part of the target on the second annular portion.
 20. Themethod of claim 19, wherein the angle at which the first annular portionis sloped is smaller than the angle at which the second annular portionis sloped.
 21. The method of claim 18, further including: directing thex-rays towards a subject; detecting x-rays passing through the subjectwith a detector; and reconstructing an image of the subject, includingaccounting for a larger flux of x-rays from the part of the target areaon the first annular portion than from the part of the target area onthe second annular portion.